Sir Alex Ferguson's apology for his attack on the referee Alan Wiley has been criticised as "half-hearted".

After a 2-2 Premier League draw with Sunderland at Old Trafford earlier this month, the Manchester United manager launched a stinging attack on the Staffordshire official. Ferguson made a series of statements about Wiley's fitness, including a belief that he took longer than necessary to book players because he "needed a rest".

The views outraged Wiley's colleagues, amid claims that the referee felt like quitting the game, because he was so upset. Ferguson has said sorry for his outburst. However, Alan Leighton, the national secretary of the union Prospect, was unimpressed.

Leighton told BBC Radio Five Live: "I think it's a half-hearted apology at best really, and it probably exacerbates the position rather than resolving it. He [Ferguson] clearly hasn't retracted the statement about Alan being unfit so it's not an apology for the main offence caused – and then he widens it to question the fitness of other referees, so he seems to be opening another can of worms which I don't think is very helpful at all.

"Referees are very fit ... they have sports scientists who test them regularly throughout the season. They don't just pass a fitness test at the start of the season. Their body fats and BMI [body mass index] are regularly monitored, there are get-togethers every two or three weeks where they are put through extensive training and testing."

Ferguson has been asked to submit his own thoughts to the Football Association by Friday. He could face a fine or a touchline ban.

Leighton said: "I think the punishment should be a Uefa-type coaching ban, which is rather more than a touchline ban. Referees always accept decisions are going to be pored over – they have no problem with legitimate criticism. What's problematic is when the integrity and key components of refereeing are being questioned in a totally unwarranted and unfounded way – and we will defend our members when they are."